Distraught Parisians and stunned tourists gazed in disbelief as the inferno raged. Thousands of onlookers lined bridges over the River Seine and along its embankments, held at a distance by a police cordon.

Witnesses said the whole island, the Ile de la Cite, was being evacuated.

"Everything is collapsing," a police officer near the scene said as the cathedral continued to burn.

World leaders expressed shock and sent condolences to the French people. President Emmanuel Macron said the whole nation was distressed. "Like all our compatriots, I am sad this evening to see this part of all of us burn," he tweeted.

A huge plume of smoke wafted across the city and ash fell over a large area. People watching gasped as the spire folded over onto itself and fell into the inferno.